The Geochemistry of the Brent Impact Structure, Ontario, Canada
Thirteen impact melt samples and basal suevitic breccias from two drill cores of the 3.8 km diameter Brent impact structure have been analyzed for major and trace elements, including PGE. In selected samples, Os, Cr, W, Sr, and Pb isotope ratios were also measured. This multi-tool approach not only characterizes the projectile responsible for the formation of the structure, but also documents the distribution and nature of the meteoritic component (s) within. The Brent simple crater is an ideal case study: the target consists of mainly gneiss with minor alnoite dykes and the structure was extensively drilled beyond its lenticular melt sheet. Using a Ni-Cr correlation, the Brent structure was previously interpreted as formed by an ordinary chondrite (OC; type L or LL) . Although the CI-normalized PGE concentrations of the melt rock show relatively flat patterns , the characteristic deviations from chondrite patterns could only be explained by fractionation of the meteoritic component. In recent years, a previously poorly constrained projectile type was characterized and proposed for the Rochechouart, Saäksjärvi, and Gardnos impact structures [3, 4]. The PGE pattern observed for Brent resembles that of IA and IIIC nonmagmatic iron meteorites (NMI). The moderately siderophile element concentrations were affected by hydrothermal alteration, but the melt zone samples retain Ni/Cr ratios not only consistent with OC, but also with IA and IIIC NMI.